For the first time since the Gulf oil spill started nearly three months ago, oil is no longer gushing into the Gulf of Mexico.

BP fixed a faulty valve on its new containment cap Thursday morning, then began its test to see if the new cap can shut in leaking oil indefinitely without harm to itself or the well bore beneath it. No oil is leaking during the test, and if the test is successful, it is possible that no oil will leak from the well ever again.

“Obviously this is an encouraging time,” Kent Wells, a senior vice president of BP, told reporters Thursday.

Officials caution that the stoppage may be only temporary. Testing of the new cap will take place over 48 hours. In that time, a team of scientists and engineers will use pressure readings and sonar to examine the condition of the well bore, which extends 13,000 feet down from the wellhead – which itself is 5,000 feet below the surface of the Gulf – to the reservoir of oil.

Engineers worry that if the well bore is damaged, oil could blow out the side of the well bore and form a new gusher elsewhere on the sea floor.

If pressure readings remain high, that will indicate that the well bore has retained its integrity – that oil is staying inside the well bore and pushing up against the cap. Low pressure readings would suggest that oil is escaping through cracks in the well bore, forcing BP to open the cap again and revert to collecting oil in surface ships.

Mr. Wells said he did not “want to create a false sense of excitement” at this time, adding: “We need to let the test move forward and learn from it and make the right decisions.”